FWIW, the oil pressure switch is in a parallel circuit to fuel pump fuse and not involved with relay. It's a backup if the relay fails.

 

thanks. what I thought, but I have no fsm except for my 91. there is a general lack of information on this, as I am sure you are aware.

three different threads in one week were looking at oil switch as a root cause for other issues
I repeated it, too, until I learned better.

the author of that bulletin was accurate, except for that.

joe

.

 

I'm still having trouble with this car. My original problem was the car was running hot. Found I had a blown head gasket, replaced it. Had the other issues discussed in this thread, got those resolved. Now I'm back where I started, still running hot. I've replaced the water pump & thermostat. Today I removed the thermostat. It still ran hot. Any suggestions?

 

I'm not 100% clear on how the ECM and oil pressure switch cooperate in turning on the fuel pump. Yes, they are in parallel.

I'm in the process of building an ECM test bench and what I see is that the ECM energizes the relay for 2 seconds and then it never turns the relay on again. I have read that the ECM turns on the relay when it sees distributor reference pulses, but I don't have that part hooked up yet, so I can't verify that.

 

Cliff, don't you see a "fuel enable" in the ecm? IF you can get to your oil switch, you can get a hint by unplugging it to see if the engine stops. I reckon you have done that already? what was the result?

I guard myself against being judgmental, but I am not as educated on ecm as you and many others are, so I follow the fsm -also ones who have verified the way it works.

I have not, because the 91 oil switch elect plug is inaccessible without removing a lot of stuff.

I suppose I could unplug the FP relay while engine is running, but that is same issue. but the key is to follow the elect diagram, if we can decipher it.

 

I just replied to another thread on a similar issue, so I've got some info.

I checked the ECM code and looked at the schematic. (Disclaimer here: I've got an '86, so that's what I'm looking at. May not apply to other years/engine combos, etc.) Anyhow, the code does not control the fuel pump at all. It's all done by the hardware (chips) in the ECM, specifically, U12 pin 10.

I just discovered another interesting item: according to the diagram on page 6E3-A-5 (engine running), if you see 12V on pin G of the ALDL connector then the fuel pump relay is NOT energized and the voltage is coming from the oil pressure switch.

 

[QUOTE=Cliff Harris;1578639275]I just replied to another thread on a similar issue, so I've got some info.

I checked the ECM code and looked at the schematic. (Disclaimer here: I've got an '86, so that's what I'm looking at. May not apply to other years/engine combos, etc.) Anyhow, the code does not control the fuel pump at all. It's all done by the hardware (chips) in the ECM, specifically, U12 pin 10.

I just discovered another interesting item: according to the diagram on page 6E3-A-5 (engine running), if you see 12V on pin G of the ALDL connector then the fuel pump relay is NOT energized and the voltage is coming from the oil pressure switch.[/QUOTE]

interesting. will check my 91 schematic.

to which code do you refer?

I am tickled that my 91 has a backup for the FP, which is oil switch. but if the relay was not enabled in run by ecm, there is no advantage

 

 

yep, my 91 fsm, exactly, AGENT86.

I think the argument can be condensed to this: granted the 91, what about previous years of c4? as I have said, ad nauseum, my logical opnion is that all were the same.

BUT, maybe not. the opinion of many others on this forum is that the early models were all run by the oil switch. I have yet to see more than one person admit that they read the schematic wrong, or produce the fsm page from an 86, whichever.

*** I correct my own misiformation. this is not an exact duplicate of my a-7 chart, since the info is on the preceding page -not on the diagnosis chart itself.

saludos, joe

 

We've got two threads going with basically the same arguments on each. I'll reply here to keep this in one place.

I hooked up an LED from ALDL pins A to G. When I drove my car it never came on. From that we can conclude that the oil pressure switch was NOT a factor and the fuel pump relay did all the work, controlled by the ECM. Also we can conclude from this that the oil pressure switch is a backup system in parallel with the fuel pump relay, as has been stated previously. I think this system was the same for these early C4s. It's really hard to be sure exactly when (or if) things changed on these cars. I do know that the same ECM (1227165) was used from '86 through '89, so most of the related parts are the same for those years. '89 deleted MAF and brought Speed Density back, so they are unique in that respect. Then we got into LT1 engines and things got REALLY different...

Side note to Joe: I worked for Lear Siegler, Inc. for 17 years. Lear sold his share of the company and used the money to start the Lear Jet company.

Here's an interesting fact also: When I got my first Corvette in 1972 (a '71), I took the passenger seat out because there was glass under it from a broken windshield. The seats had the Lear Siegler logo (Circle with LSI inside) on metal tags on them. I believe all Corvette seats from that era and for many years after were made by LSI (possibly even up until today, but I don't know for sure about that).

 

glad we finally agree, Cliff! Agent 86 is, too! also, glad that at least one person has the know-how to verify this, and is honest enuff to say his info was at least partially incorrect..

I am certain that someone will be on here before long citing the oil switch as a fuel delivery issue! I have read it many times, from people who should know better -esp if they build race engines. I was a drag racer in a former lifetime, but have only been exploring the ecm functions of my car since June -no expert by any means. I think the key to finding the one root cause of anything is to be self-critical of our assumptions, as well as the info from others..

I am a "historical fiction" writer, for therapy, not fame, and based some stories around the fabulous Learjet. that Bill Lear, and his son, Jr, had some kind of special brains! I bought the Lj23-24-25 flight training manual to understand how everything worked. Fascinating stuff.

another interesting morsel: one of my stories referred to a supplier's metal tag under the seats -which I had NO idea existed, until now.

mucho gusto!

saludos, joe